The invention relates to capacitance-based position sensors, and more particularly to capacitance-based position sensors where relative displacements occurring between objects are determined in part by applying time-varying input signals to a capacitor having a value that varies with relative displacement of the objects.
Capacitance-based position sensors are widely known. Many such sensors employ a variable capacitor having a value that varies with relative position of a pair of objects. In these systems, the relative position of the objects can be determined by measuring the capacitance.
A basic design issue arises in these systems from the fact that a capacitor acts as an open circuit when non-time-varying signals, such as DC voltages, are applied to the capacitor. Therefore, to produce a measurable response, that is, a response from which capacitance can be determined, a time-varying signal is often applied as an input. Frequently, this input signal is a high frequency sinusoidal carrier.
Application of such a time-varying input yields a corresponding time-varying output from the position-dependent capacitance. In addition to varying as a function of time, the output varies with the value of the capacitance. Due to the time-varying, and often high frequency, nature of the output, additional processing is normally necessary to extract the desired information (e.g., the value of the capacitance).
The structures and/or methods used in existing systems to extract this information, though sometimes adequate, are not suitable for a large number of applications in which capacitance-based position sensing can be used. For example, capacitance-based position sensing is often used in very small, precise applications, such as in micro electromechanical systems (MEMS), and/or in applications involving a number of different relative movements. In these situations, and in a variety of other applications, simplicity of design is an important consideration. In many existing systems, however, a relatively large amount of circuitry is needed to extract the desired information from the time-varying output of the variable capacitor. This circuitry is often fairly precise and complex. In addition, it can consume a large amount of physical space and generate excessive heat.
The present invention provides a capacitance-based position sensor, which may be used to sense relative position of a pair of objects. The sensor includes capacitor circuitry having a capacitor that varies in capacitance with relative position of the pair of objects. The sensor also includes a source configured to apply a time-varying input to the capacitor circuitry. The capacitor circuitry is configured to yield a time-varying, capacitance-dependent output in response to application of the time-varying input. The sensor further includes an integrator configured to integrate, for a predetermined time period, a received signal that is based upon the time-varying, capacitance-dependent output from the capacitor circuitry. The integration yields an integrator output which is proportional to the value of the capacitance. This output is usable to determine relative position of the objects. In some embodiments, the sensor is used to sense relative position of a computer storage medium and an access device that is configured to selectively access storage locations provided on the storage medium.